The Birdy is really nice to ride, I also have a Brompton I personal think the Birdy is a better ride, the 18 inch Wheels and the front Suspension make a big difference.
I bought mine secondhand as it was in really good condition, it does not fold down as small as the Brompton,but folds down small enough to pop into the car.which is great,I think if you are commuting a lot on Buses or Train's the Brompton is better bet.
The Birdy has 21 gears compared to the Brompton 6 gears,which is handy when riding in hilly terrain.